13C Labelling Study of the Interconversion of Ethylbenzene, 7-Methylcycloheptatriene and p-Xylene Ions

Article abstract of DOI:10.1002/oms.1210170802

The isomerization of the molecular ions of ethylbenzene, 7-methylcycloheptatriene and p-xylene by skeletal rearrangement prior to the formation of (1+) ions has been investigated by using 13C labelled compounds.The results obtained for ions generated by 70 eV and 12 eV electron impact, and fragmenting in the ion source, the 1st field free region and 2nd field free region, respectively, are compared with those obtained from D labelled derivatives.It is shown that at long reaction times metastable p-xylene ions lose a methyl radical after scrambling of all C atoms and H atoms, while the unstable molecular ions in the ion source react by specific loss of one of the methyl substituents.Both unstable and metastable ethylbenzene ions fragment by two competing mechanisms, one corresponding to specific loss of the terminal methyl group, and the other involving scrambling of all C and H atoms.These results are discussed by use of a dynamic model developed for the mutual interconversion and fragmentation of the molecular ions of ethylbenzene, methylcycloheptatriene and p-xylene.The experimental results can be explained by an equilibrium between metastable methylcycloheptatriene ions and p-xylene ions with suffucient energy for skeletal rearrangement, while about 40 percent of the metastable ethylbenzene ions fragment after rearrangement to methylcycloheptatriene ions and about 60 percent of the ethylbenzene ions rearrange further to xylene ions before fragmentation.Metastable methylcycloheptatriene ions, mainly lose a methyl group without a skeletal rearrangement, however, because the rearranged ions are kinetically trapped as 'stable' xylene ions or ethylbenzene ions.